In signal processing it is sometimes necessary to utilize a value of a received input signal as it existed at a prior time in order to perform a desired control or information processing function. Digital systems generally sample incoming signals at a very high rate which is on the order of many times a second. In known systems which perform control or information processing functions based on received signals, a large amount of storage is required in digital systems and a large number of delay elements are required in analog systems if the value of the incoming signal as it existed in the past is needed. For example, an electronic automatic temperature controller (EATC) used on an automotive vehicle utilizes a temperature sensor to generate a signal indicative of ambient temperature for use by the EATC. Unless the sensor is placed in a position far removed from the engine compartment, heat generated by the engine, exhaust or other components is likely to affect the temperature detected by the sensor. When the sensor is located in the engine compartment at a position to be exposed to air entering the engine compartment, it has been found to provide an accurate reading when the vehicle is in motion. However, when the vehicle is standing the temperature sensor gives a much higher reading than normal because of the increased air temperature in the engine compartment. In such a circumstance, it is helpful to have the value of temperature as it existed several minutes in the past, such as before the vehicle stopped, so that the EATC can accurately determine the existing ambient temperature. With certain finite impulse response filters, if a five minute delay is desired for a signal sampled every second, a system utilizing a finite impulse response filter of known type would need 300 storage or delay elements. One solution is for the EATC to utilize a vehicle velocity signal which is indicative of vehicle velocity in order to determine when the signal from the temperature sensor should be ignored. However, if the vehicle velocity signal is not already being used by the EATC, then use of such a signal adds to the cost and physical complexity of the EATC. In addition, even if the vehicle velocity signal is available to the EATC, an accurate value of ambient air temperature is still required. If the vehicle is stopped momentarily, then a temperature value indicative of the temperature before the vehicle stopped, will still be required.
Accordingly, there is a need for a filter which generates a delayed value of an input signal without requiring the large number of storage or delay elements required by known systems.